Sorry for this question, it must have been asked a zillion times, but I still can’t find the answer 
Below you will find a partial shot of my model of Petr Velikii, a Russian ironclad of 1872.
As you can see the after turret is smoothed, the forward one isn’t. There are weird shading effects on the smoothed surfaces.
Removing duplicate vertices and recalculating normals doesn’t help. Can you ?
Thanks in advance,
Thierry
Check your normals. There must be faces pointing outside and inside side by side. (Use CTRL+N to recalculate the normals of the selected faces.) Next, check your topology, smooth shading doesn’t like much triangles, Ngons and anything that isn’t a nice square quad.
Avec un coucou de Normandie. 
Looks like you have a bunch of triangles around the barrels, which will cause shading artifacts. I’m only guessing, but did you boolean the holes into the turret?
To kaluura and ajm: you were both right! The gun embrasures had been created by booleans, and these annoying artefacts disappeared when I deleted the faces around the embrasures and recreated them with “nice” quads.
Quite a number of things to learn about smoothing, I guess…
Thank you so much,
Thierry
Avec un petit coucou de Bretagne 
